1. Field of the Invention
The present invention relates to an exposure apparatus having a projection optical system for projecting a pattern of a first object onto a photosensitive substrate etc. as a second object, and more particularly to a projection optical system suitably applicable to projection exposure of a pattern for semiconductor or liquid crystal formed on a reticle (mask) as the first object onto the substrate (semiconductor wafer, plate, etc.) as the second object.
2. Related Background Art
As the patterns of integrated circuits become finer and finer, the resolving power required for the exposure apparatus used in printing of wafer also becomes higher and higher. In addition to the improvement in resolving power, the projection optical systems of the exposure apparatus are required to decrease image stress. In order to get ready for the finer tendency of transfer patterns, light sources for exposure have recently been changing from those emitting the light of exposure wavelength of the g-line (436 nm) to those emitting the light of exposure wavelength of the i-line (365 nm) that are mainly used at present. Further, a trend is to use light sources emitting shorter wavelengths, for example the excimer laser (KrF:248 nm, ArF:193 nm).
Here, the image stress includes those due to bowing etc. of the printed wafer on the image side of projection optical system and those due to bowing etc. of the reticle with circuit pattern etc. written therein, on the object side of projection optical system, as well as distortion caused by the projection optical system.
With a recent further progress of fineness tendency of transfer patterns, demands to decrease the image stress are also becoming harder.
Then, in order to decrease effects of the wafer bowing on the image stress, the conventional technology has employed the so-called image-side telecentric optical system that located the exit pupil position at a farther point on the image side of projection optical system.
On the other hand, the image stress due to the bowing of reticle can also be reduced by employing a so-called object-side telecentric optical system that locates the entrance pupil position of projection optical system at a farther point from the object plane, and there are suggestions to locate the entrance pupil position of projection optical system at a relatively far position from the object plane as described. Examples of those suggestions are described for example in Japanese Laid-open Patent Applications No. 63-118115 and No. 5-173065 and U.S. Pat. No. 5,260,832.
An object of the present invention is to provide a high-performance projection optical system which can achieve the bitelecentricity in a compact design as securing a wide exposure area and a large numerical aperture and which can be well corrected for aberrations, particularly which can be very well corrected for distortion. The projection optical system can be applied to an exposure apparatus.
To achieve the above object, an exposure apparatus according to the present invention comprises at least a wafer stage allowing a photosensitive substrate to be held on a main surface thereof, an illumination optical system for emitting exposure light of a predetermined wavelength and transferring a predetermined pattern of a mask (reticle) onto the substrate, a projection optical system provided between a first surface on which the mask as a first object is disposed and a second surface on which a surface of the substrate as a second object is corresponded, for projecting an image of the pattern of the mask onto the substrate. The illumination optical system includes as alignment optical system for adjusting a relative positions between the mask and the wafer, and the mask is disposed on a reticle stage which is movable in parallel with respect to the main surface of the wafer stage. The projection optical system has a space permitting an aperture stop to be set therein. The photosensitive substrate comprises a wafer such as a silicon wafer or a glass plate, etc., and a photosensitive material such as a photoresist or the like coating a surface of the wafer. In particular, as shown in FIG. 1, the projection optical system includes a first lens group (G1) with a positive refracting power, a second lens group (G2) with a negative refracting power, a third lens group (G3) with a positive refracting power, a fourth lens group (G4) with a negative refracting power, a fifth lens group (G5) with a positive refracting power, and a sixth lens group (G6) with a positive refracting power in order from the side of the first object (for example, a mask).
The second lens group (G2) comprises a front lens (L2F) with a negative refracting power disposed as closest to the first object and shaded with a concave surface to the second object, a rear lens (L2R) of a negative meniscus shape disposed as closest to the substrate and shaped with a concave surface to the mask, and an intermediate lens group (G2M) disposed between the front lens (L2F) and the rear lens (L2R). In particular, the intermediate lens group (G2M) has a first lens (LM1) with a positive refracting power, a second lens (LM2) with a negative refracting power, and a third lens (LM3) with a negative refracting power in order from the side of the first object.
Further, the projection optical system according to the present invention is arranged to satisfy the following conditions (1) to (6) when f1 is a focal length of the first lens group (G1), f2 is a focal length of the second lens group (G2), f3 is a focal length of the third lens group (G3), f4 is a focal length of the fourth lens group (G4), f5 is a focal length of the fifth lens group (G5), f6 is a focal length of the sixth lens group (G6), and L is a distance from the first object to the second object:
(1) f1/L less than 0.8
(2) xe2x88x920.033 less than f2/L
(3) 0.01 less than f3/L less than 1.0
(4) f4/L less than xe2x88x920.005
(5) 0.01 less than f5/L less than 0.9
(6) 0.02 less than f6/L less than 1.6.
The projection optical system is so arranged as to have at least the first lens group (G1) with positive refracting power, the second lens group (G2) with negative refracting power, the third lens group (G3) with positive refracting power, the fourth lens group (G4) with negative refracting power, the fifth lens group (G5) with positive refracting power, and the sixth lens group (G6) with positive refracting power in the named order from the first object side.
First, the first lens group (G1) with positive refracting power contributes mainly to a correction of distortion while maintaining telecentricity, and specifically, the first lens group (G1) is arranged to generate a positive distortion to correct in a good balance negative distortions caused by the plurality of lens groups located on the second object side after the first lens group (G1). The second lens group (G2) with negative refracting power and the fourth lens group (G4) with negative refracting power contribute mainly to a correction of Petzval sum to make the image plane flat. The two lens groups of the second lens group (G2) with negative refracting power and the third lens group (G3) with positive refracting power form an inverse telescopic system to contribute to guarantee of back focus (a distance from an optical surface such as a lens surface closest to the second object in the projection optical system to the second object) in the projection optical system. The fifth lens group (G5) with positive refracting power and the sixth lens group (G6) similarly with positive refracting power contribute mainly to suppressing generation of distortion and suppressing generation particularly of spherical aberration as much as possible in order to fully support high NA structure on the second object side.
Based on the above arrangement, the front lens (L2F) with the negative refracting power disposed as closest to the first object in the second lens group (G2) and shaped with the concave surface to the second object contributes to correction for curvature of field and coma, and the rear lens (L2R) of the negative meniscus shape disposed as closest to the second object in the second lens group (G2) and shaped with the concave surface to the first object contributes mainly to correction for coma. The rear lens (L2R) also contributes to correction for curvature of field. Further, in the intermediate lens group (G2M) disposed between the front lens (L2F) and the rear lens (L2R), the first lens (LM1) with the positive refracting power contributes to correction for negative distortion generated by the second lens (LM2) and third lens (LM3) of the negative refracting powers greatly contributing to correction for curvature of field.
Condition (1) defines an optimum ratio between the focal length f1 of the first lens group (G1) with the positive refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). This condition (1) is mainly for well-balanced correction for distortion.
Above the upper limit of condition (1), large negative distortion will appear. In order to achieve a compact design as securing a reduction magnification and a wide exposure area and to achieve good correction for distortion, the upper limit of condition (1) is preferably set to 0.14, as f1/L less than 0.14. In order to suppress appearance of spherical aberration of pupil, the lower limit of condition (1) is preferably set to 0.02, as 0.02 less than f1/L.
Condition (2) defines an optimum ratio between the focal length f2 of the second lens group (G2) with the negative refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). This condition (2) is a condition for achieving a compact design as securing a wide exposure region and achieving good correction for Petzval sum.
Here, below the lower limit of condition (2), it becomes difficult to achieve the compact design as securing the wide exposure region and positive Petzval sum will appear, thus not preferred. In order to achieve further compact design or superior correction for Petzval sum, the lower limit of condition (2) is preferably set to xe2x88x920.032, as xe2x88x920.032 less than f2/L. In order to suppress appearance of negative distortion, the upper limit of condition (2) is preferably set to xe2x88x920.005, as f2/L less than xe2x88x920.005.
Condition (3) defines an optimum ratio between the focal length f3 of the third lens group (G3) with the positive refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). Here, below the lower limit of condition (3), the refractive power of the second lens group (G2) or the fourth lens group (G4) becomes too strong, resulting in giving rise to negative distortion and coma in the second lens group (G2) or giving rise to coma in the fourth lens group (G4). On the other hand, above the upper limit of condition (3), the refractive power of the second lens group (G2) or the fourth lens group (G4) becomes too weak, failing to well correct Petzval sum.
Condition (4) defines an optimum ratio between the focal length f4 of the fourth lens group (G4) with the negative refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.).
Here, above the upper limit of condition (4), coma will appear, thus not preferred. Further, in order to suppress appearance of coma, the upper limit of condition (4) is preferably set to xe2x88x920.047, as f4/L less than xe2x88x920.047.
In order to well correct spherical aberration, the lower limit of condition (4) is preferably set to xe2x88x920.098, as xe2x88x920.098 less than f4/L.
Condition (5) defines an optimum ratio between the focal length f5 of the fifth lens group (G5) with the positive refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). This condition (5) is for achieving well-balanced correction for spherical aberration, distortion, and Petzval sum as maintaining a large numerical aperture. Below the lower limit of this condition (5), the refracting power of the fifth lens group (G5) becomes too strong, resulting in giving rise to great negative spherical aberration in addition to negative distortion in the fifth lens group (G5). Above the upper limit of this condition (5), the refracting power of the fifth lens group (G5) becomes too weak, which inevitably weakens the refracting power of the fourth lens group (G4) with the negative refracting power. As a consequence, Petzval sum will not be well corrected.
Condition (6) defines an optimum ratio between the focal length f6 of the sixth lens group (G6) with the positive refracting power and the distance (object-to-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). This condition (6) is for suppressing appearance of higher-order spherical aberration and negative distortion as maintaining a large numerical aperture. Below the lower limit of this condition (6), the sixth lens group (G6) itself gives rise to great negative distortion; above the upper limit of this condition (6), higher-order spherical aberration will appear.
On the basis of the above composition it is preferred that when I is an axial distance from the first object to a first-object-side focal point F of the entire projection optical system and L is the distance from the first object to the second object, the following condition be satisfied:
1.0 less than I/L.xe2x80x83xe2x80x83(7)
The condition (7) defines an optimum ratio between the axial distance I from the first object to the first-object-side focal point F of the entire projection optical system and the distance (object-image distance) L from the first object (reticle etc.) to the second object (wafer etc.). Here, the first-object-side focal point F of the entire projection optical system means an intersecting point of outgoing light from the projection optical system with the optical axis after collimated light beams are let to enter the projection optical system on the second object side in the paraxial region with respect to the optical axis of the projection optical system and when the light beams in the paraxial region are outgoing from the projection optical system.
Below the lower limit of this condition (7) the first-object-side telecentricity of the projection optical system will become considerably destroyed, so that changes of magnification and distortion due to an axial deviation of the first object will become large. As a result, it becomes difficult to faithfully project an image of the first object at a desired magnification onto the second object. In order to fully suppress the changes of magnification and distortion due to the axial deviation of the first object, the lower limit of the above condition (7) is preferably set to 1.7, i.e., 1.7 less than I/L. Further, in order to correct a spherical aberration and a distortion of the pupil both in a good balance while maintaining the compact design of the projection optical system, the upper limit of the above condition (7) is preferably set to 6.8, i.e., I/L less than 6.8.
It is also preferred that the fourth lens group (G4) have a front lens group disposed as closest to the first object and a rear lens group disposed as closest to the second object, that an intermediate lens group having a first negative lens (L43) and a second negative lens (L44) in order from the side of the first object be disposed between the front lens group in the fourth lens group (G4) and the rear lens group in the fourth lens group (G4), that the front lens group have two negative meniscus lenses (L41, L42) each shaped with a concave surface to the second object, that the rear lens group has a negative lens (L46) with a concave surface to the first object, and that when f4A is a focal length of the first negative lens (L43) in the fourth lens group (G4) and f4B is a focal length of the second negative lens (L44) in the fourth lens group (G4), the following condition be satisfied:
0.05 less than f4A/f4B less than 20.xe2x80x83xe2x80x83(8)
Below the lower limit of condition (8), the refractive power of the first negative lens (L43) becomes strong relative to the refractive power of the second negative lens (L44), so that the first negative lens (L43) will give rise to higher-order spherical aberration and higher-order coma. In order to suppress appearance of the higher-order spherical aberration and higher-order coma, the lower limit of the above condition (8) is preferably set to 0.1, as 0.1 less than f4A/f4B. On the other hand, above the upper limit of condition (8), the refracting power of the second negative lens (L44) becomes strong relative to the refracting power of the first negative lens (L43), so that the second negative lens (L44) will give rise to higher-order spherical aberration and higher-order coma. In order to further suppress appearance of higher-order spherical aberration and higher-order coma, the upper limit of the above condition (8) is preferably set to 10, as f4A/f4B less than 10.
It is also preferred that when r2Ff is a radius of curvature of a first-object-side surface of the front lens (L2F) and r2Fr is a radius of curvature of a second-object-side surface of the front lens (L2F), the front lens (L2F) in the second lens group (G2) satisfy the following condition:
1.00xe2x89xa6(r2Ffxe2x88x92r2Fr)/(r2Ff+r2Fr) less than 5.0.xe2x80x83xe2x80x83(9)
Below the lower limit of this condition (9), sufficient correction for spherical aberration of pupil becomes impossible, thus not preferred. On the other hand, above the upper limit of this condition (9), coma will appear, thus not preferred.
It is also preferred that the fourth lens group (G4) have a front lens group having a negative lens (L41) disposed as closest to the first object and shaped with a concave surface to the second object, and a rear lens group having a negative lens (L46) disposed as closest to the second object and shaped with a concave surface to the first object, that an intermediate lens group having at least a negative lens (L44) and a positive lens (L45) with a convex surface adjacent to a concave surface of the negative lens (L44) be disposed between the front lens group in the fourth lens group (G4) and the rear lens group in the fourth lens group (G4), and that when r4N is a radius of curvature of the concave surface of the negative lens (L44) in the intermediate lens group and r4P is a radius of curvature of the convex surface of the positive lens (L45) in the intermediate lens group, the following condition be satisfied:
xe2x88x920.9 less than (r4Nxe2x88x92r4P)/(r4N+r4P) less than 0.9,xe2x80x83xe2x80x83(10)
provided that when L is the distance from the first object to the second object, the concave surface of the negative lens (L44) in the intermediate lens group or the convex surface of the positive lens (L45) in the intermediate lens group satisfies at least one of the following conditions:
|r4N/L| less than 2.0xe2x80x83xe2x80x83(11) 
|r4P/L| less than 2.0.xe2x80x83xe2x80x83(12)
Conditions (10) to (12) define an optimum configuration of a gas lens formed by the concave surface of the negative lens (L44) in the intermediate lens group and the convex surface of the positive lens (L45) in the intermediate lens group. When condition (11) or (12) is satisfied, this gas lens can correct higher-order spherical aberration. For further correction of higher-order spherical aberration, the upper limits of condition (11) and condition (12) are preferably set to 0.8, as |r4N/L| less than 0.8 and |r4P/L| less than 0.8. Here, above the upper limit or below the lower limit of condition (10), coma will appear, thus not preferred. If neither condition (11) nor condition (12) is satisfied, correction for higher-order spherical aberration is impossible even if condition (10) is satisfied, thus not preferred.
It is also preferred that when f22 is a focal length of the second lens (LM2) with the negative refracting power in the second lens group (G2) and f23 is a focal length of the third lens (LM3) with the negative refracting power in the second lens group (G2), the following condition be satisfied:
0.1 less than f22/f23 less than 10.xe2x80x83xe2x80x83(13)
Below the lower limit of the condition (13) the refracting power of the second negative lens (LM2) becomes strong relative to the refracting power of the third negative lens (LM3), so that the second negative lens (LM2) generates a large coma and a large negative distortion. In order to correct the negative distortion in a better balance, the lower limit of the above condition (13) is preferably set to 0.7, i.e., 0.7 less than f22/f23. Above the upper limit of this condition (13) the refracting power of the third negative lens (LM3) becomes strong relative to the refracting power of the second negative lens (LM2), so that the third negative lens generates a large coma and a large negative distortion. In order to correct the negative distortion in a better balance while well correcting the coma, the upper limit of the above condition (13) is preferably set to 1.5, i.e., f24/f23 less than 1.5.
It is also preferred that the fifth lens group (G5) have a negative meniscus lens (for example, L54), and a positive lens (for example, L53) disposed as adjacent to a concave surface of the negative meniscus lens and having a convex surface opposed to the concave surface of the negative meniscus lens and that when r5n is a radius of curvature of the concave surface of the negative meniscus lens in the fifth lens group (G5) and r5P is a radius of curvature of the convex surface, opposed to the concave surface of the negative meniscus lens, of the positive lens disposed as adjacent to the concave surface of the negative meniscus lens in the fifth lens group (G5), the following condition be satisfied:
0 less than (r5Pxe2x88x92r5n)/(r5P+r5n) less than 1.xe2x80x83xe2x80x83(14)
In this case, it is preferred that the negative meniscus lens (for example, L54) and the positive lens (L53) adjacent to the concave surface of the negative meniscus lens be disposed between at least one positive lens (for example, L52) in the fifth lens group G5 and at least one positive lens (for example, L55) in the fifth lens group (G5).
In this case, in order to suppress the negative distortion without generating the higher-order spherical aberrations in the lens (L61) located closest to the first object in the sixth lens group (G6), it is desirable that the lens surface closest to the first object have a shape with a convex surface to the first object and that the following condition be satisfied when a radius of curvature on the second object side, of the negative lens (L58) placed as closest to the second object in the fifth lens group (G5) is r5R and a radius of curvature on the first object side, of the lens (L61) placed as closest to the first object in the sixth lens group (G6) is r6F.
xe2x88x920.90 less than (r5Rxe2x88x92r6F)/(r5R+r6F) less than xe2x88x920.001xe2x80x83xe2x80x83(15)
This condition (15) defines an optimum shape of a gas lens formed between the fifth lens group (G5) and the sixth lens group (G6). Below the lower limit of this condition (15) a curvature of the second-object-side concave surface of the negative lens (L58) located closest to the second object in the fifth lens group (G5) becomes too strong, thereby generating higher-order comas. Above the upper limit of this condition (15) refracting power of the gas lens itself formed between the fifth lens group (G5) and the sixth lens group (G6) becomes weak, so that a quantity of the positive distortion generated by this gas lens becomes small, which makes it difficult to well correct a negative distortion generated by the positive lens in the fifth lens group (G5). In order to fully suppress the generation of higher-order comas, the lower limit of the above condition (15) is preferably set to xe2x88x920.30, i.e., xe2x88x920.30 less than (r5Rxe2x88x92r6F)/(r5R+r6F).
Also, it is further preferable that the following condition be satisfied when a lens group separation between the fifth lens group (G5) and the sixth lens group (G6) is d56 and the distance from the first object to the second object is L.
d56/L less than 0.017xe2x80x83xe2x80x83(16)
Above the upper limit of this condition (16), the lens group separation between the fifth lens group (G5) and the sixth lens group (G6) becomes too large, so that a quantity of the positive distortion generated becomes small. As a result, it becomes difficult to correct the negative distortion generated by the positive lens in the fifth lens group (G5) in a good balance.
Also, it is more preferable that the following condition be satisfied when a radius of curvature of the lens surface closest to the first object in the sixth lens group (G6) is r6F and an axial distance from the lens surface closest to the first object in the sixth lens group (G6) to the second object is d6.
0.50 less than d6/r6F less than 1.50xe2x80x83xe2x80x83(17)
Below the lower limit of this condition (17), the positive refracting power of the lens surface closest to the first object in the sixth lens group (G6) becomes too strong, so that a large negative distortion and a large coma are generated. Above the upper limit of this condition (17), the positive refracting power of the lens surface closest to the first object in the sixth lens group (G6) becomes too weak, thus generating a large coma. In order to further suppress the generation of coma, the lower limit of the condition (17) is preferably set to 0.84, i.e., 0.84 less than d6/r6F.
Also, it is to be more desired that said fifth lens group (G5) have a negative lens (L58) placed as closest to the second object and having a concave surface opposed to the second object and that the following condition be satisfied when a radius of curvature on the first object side in the negative lens (L58) closest to the second object in said fifth lens group (G5) is r5F and a radius of curvature on the second object side in the negative lens (L58) closest to the second object in said fifth lens group (G5) is r5R:
0.30 less than (r5Fxe2x88x92r5R)/(r5F+r5R) less than 1.28.xe2x80x83xe2x80x83(18)
Below the lower limit of this condition (18), it becomes difficult to correct both the Petzval sum and the coma; above the upper limit of this condition (18), large higher-order comas appear, which is not preferable. In order to further prevent the generation of higher-order comas, the upper limit of the condition (18) is preferably set to 0.93, i.e., (r5Fxe2x88x92r5R)/(r5F+r5R) less than 0.93.
It is more desired that when f21 is a focal length of the first lens (LM1) with the positive refracting power in the intermediate lens group (G2M) in the second lens group (G2) and L is the distance from the first object to the second object, the following condition be satisfied:
0.230 less than f21/L less than 0.40.xe2x80x83xe2x80x83(19)
Below the lower limit of condition (19), positive distortion will appear; above the upper limit of condition (19), negative distortion will appear, either of which is thus not preferred. Further, in order to further correct the negative distortion, the second-object-side lens surface of the first lens (LM1) is preferably formed in a lens configuration shaped with a convex surface facing the second object.
It is also preferred that when f2F is a focal length of the front lens (L2F) with the negative refracting power disposed as closest to the first object in the second lens group (G2) and shaped with the concave surface to the second object and f2R is a focal length of the rear lens (L2R) with the negative refracting power disposed as closest to the second object in the second lens group (G2) and shaped with the concave surface to the first object, the following condition be satisfied:
0xe2x89xa6f2F/f2R less than 18.xe2x80x83xe2x80x83(20)
Also, the front lens (L2F) and the rear lens (L2R) in the second lens group (G2) preferably satisfy the following condition when the focal length of the front lens (L2F) placed as closest to the first object in the second lens group (G2) and having the negative refracting power with a concave surface to the second object is f2F and the focal length of the rear lens (L2R) placed as closest to the second object in the second lens group (G2) and having the negative refracting power with a concave surface to the second object is f2R.
0xe2x89xa6f2F/f2R less than 18xe2x80x83xe2x80x83(20)
The condition (20) defines an optimum ratio between the focal length f2R of the rear lens (L2R) in the second lens group (G2) and the focal length f2F of the front lens (L2F) in the second lens group (G2). Below the lower limit and above the upper limit of this condition (20), a balance is destroyed for refracting power of the first lens group (G1) or the third lens group (G3), which makes it difficult to correct the distortion well or to correct the Petzval sum and the astigmatism simultaneously well.
In order to further well correct Petzval sum, the intermediate lens group (G2M) in the second lens group (G2) preferably has a negative refracting power.
For the above lens groups to achieve satisfactory aberration correction functions, specifically, they are desired to be constructed in the following arrangements.
First, for the fist lens group (G1) to have a function to suppress appearance of higher-order distortion and appearance of spherical aberration of pupil, the first lens group (G1) preferably has at least two positive lenses; for the third lens group (G3) to have a function to suppress degradation of spherical aberration and Petzval sum, the third lens group (G3) preferably has at least three positive lenses; further, for the fourth lens group (G4) to have a function to suppress appearance of coma as correcting Petzval sum, the fourth lens group (G4) preferably has at least three negative lenses. For the fifth lens group (G5) to have a function to suppress appearance of negative distortion and spherical aberration, the fifth lens group (G5) preferably has at least five positive lenses; further, for the fifth lens group (G5) to have a function to correct negative distortion and Petzval sum, the fifth lens group (G5) preferably has at least one negative lens. For the sixth lens group (G6) to effect focus on the second object so as not to give rise to large spherical aberration, the sixth lens group (G6) preferably has at least one positive lens.
For further compact design, the intermediate lens group in the second lens group desirably comprises only two negative lenses.
For the sixth lens group (G6) to have a function to further suppress appearance of negative distortion, the sixth lens group (G6) is preferably arranged to comprise three or less lenses including at least one lens surface satisfying the following condition (21).
1/|"PHgr"L| less than 20xe2x80x83xe2x80x83(21)
where
"PHgr": a refractive power of the lens surface; and
L: the distance (object-to-image distance) from the first object to the second object.
The refractive power of lens surface, stated here, is given by the following equation where r is a radius of curvature of the lens surface, n1 a refractive index of a medium on the first object side of the lens surface, and n2 a refractive index of a medium on the second object side of the lens surface.
"PHgr"=(n2xe2x88x92n1)/r
Here, if there are four or more lenses having the lens surface satisfying this condition (21), the number of lens surfaces with some curvature, located near the second object, becomes increased, which generates the distortion, thus not preferable.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art form this detailed description.